Displays fabricated using OLEDs (organic light emitting displays) provide a number of advantages over other flat panel technologies. They are bright, colorful, fast-switching, provide a wide viewing angle, and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colors which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/2 1935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multi-colored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.
FIG. 1a shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1a). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminum, or a layer of aluminum sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are available from Corning, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
A substantially transparent hole transport layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from Bayer AG of Germany. In a typical polymer-based device the hole transport layer 106 may comprise around 200 nm of PEDOT; a light emitting polymer layer 108 is typically around 70 nm in thickness.
These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.
The cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapor deposition) covered with a thicker, capping layer of aluminum. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved or enhanced through the use of cathode separators (not shown in FIG. 1a).
The same basic structure may also be employed for small molecule devices.
Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
To illuminate the OLED power is applied between the anode and cathode, represented in FIG. 1a by battery 118. In the example shown in FIG. 1a light is emitted through the transparent anode 104 and the substrate 102 and the cathode is generally reflective; such devices are referred to as “bottom emitters.” Devices which emit through the cathode (“top emitters”) may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
Referring now to FIG. 1b, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of FIG. 1a are indicated by like reference numerals. As shown the hole transport 106 and electroluminescent 108 layers are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal can 111, attached by UV-curable epoxy glue 113 onto the anode metal layer 104, small glass beads within the glue preventing the metal can touching and shorting out the contacts. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
Considerable effort has been dedicated to the realization of a full-color, all plastic screen. The major challenges to achieving this goal have been: (1) access to conjugated polymers emitting light of the three basic colors red, green and blue; and (2) the conjugated polymers must be easy to process and fabricate into full-color display structures. PLED devices show great promise in meeting the first requirement, since manipulation of the emission color can be achieved by changing the chemical structure of the conjugated polymers. However, while modulation of the chemical nature of conjugated polymers is often easy and inexpensive on the lab scale it can be an expensive and complicated process on the industrial scale. The second requirement of the easy processability and build-up of full-color matrix devices raises the question of how to micro-pattern fine multicolor pixels and how to achieve full-color emission. Inkjet printing and hybrid inkjet printing technology have attracted much interest for the patterning of PLED devices (see, for example, R. F. Service, Science 1998, 279, 1135; Wudl et al., Appl. Phys. Lett. 1998, 73, 2561; J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660; and T. R. Hebner, C. C. Wu, D. Marcy, M. L. Lu, J. Sturm, Appl. Phys. Lett. 1998, 72, 519).
In order to contribute to the development of a full-color display, conjugated polymers exhibiting direct color-tuning, good processability and the potential for inexpensive large-scale fabrication have been sought. The step-ladder polymer poly-2,7-fluorenes have been the subject of much research into blue-light emitting polymers (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629; J. S. Kim, R. H. Friend, and F. Cacialli, Appl. Phys. Lett. 1999, 74, 3084; WO-A-00/55927 and M. Bernius et al., Adv. Mater., 2000, 12, No. 23, 1737). This class of conjugated polymers possesses excellent processability, endowed by the attachment of solubilizing groups (particularly aryl groups) at the remote C-9 position, without hampering the extended conjugation and therefore leading to high fluorescence quantum yields in the solid state (see, for example, Q. Pei, Y. Yang, J. Am. Chem. Soc. 1996, 118, 7416). Other benefits of poly-9,9-diaryl-substituted fluorenes are their excellent thermal (Td>400° C.) and chemical stability and their good film forming properties.
The process to make homo- and copolymers based on 9,9-disubstituted fluorene monomers depends on the metal-mediated cross coupling of both AA-BB and AB type monomers. There is now a considerable prior art in the field. Such copolymers may be made by the cross coupling of dibromo-substituted monomers by contacting them with a Ni(0) catalyst formed in situ from a Ni(II) salt (the Yamamoto coupling, Progress in Polymer Science, Vol. 17, p. 1153, 1992) (E. P. Woo et al., U.S. Pat. Nos. 5,708,130; 5,962,631). A Pd(0) mediated cross coupling between arylboronic acids and esters and aryl or vinyl halides (the Suzuki coupling, A. Suzuki et al., Synth. Commun., 1981, 11, 513) has been developed in the presence of a phase transfer catalyst and an inorganic base to make relatively high quality poly(fluorene) derivatives for applications as PLEDs (M. Inbasekaran, U.S. Pat. No. 5,777,070). Extension to various comonomers having hole transporting properties has also been realised (WO-A-99/54385). In a further development a combination of a catalyst and a base was selected to convert the boron functional groups into —BX3— where X is either F or OH (WO-A-00/53656).
One problem associated with the synthesis of these homo- and copolymers is the fact that the corresponding 2,7-derivatized monomers such as 2,7-dibromo-substituted 9,9-diarylfluorenes are difficult to synthesize. There are a number of methods currently used, none of which are very satisfactory due to their relatively poor yields and difficulties in scaling up the processes. For example, one of the most commonly used methods currently employed for the synthesis of 2,7-dibromo-9,9-diphenylfluorene is the following 5-step process disclosed in DL 198 467 67:

The yield of only 40% over 5 steps is poor and the technique itself lacks flexibility to allow different types of functionality to be introduced at the 9-position. It is highly desirable to develop a quicker, more flexible process that would enable diarylfluorene monomers and analogs thereof to be synthesized in fewer steps at a higher yield, said process being one that can readily be applied at an industrial scale.